Damage to, or loss of, photoreceptors (PRs) in the eye, and/or damage to layers of the retina that prevents PR transmission to the brain, can lead to blindness. Photoreceptors detect light and stimulate downstream neurons in the retina. Around 1 million people in the United States alone suffer profound vision loss, with another 2.4 million having some degree of visual impairment. As the U.S. population continues to age, it is likely that the total number of affected individuals will increase, possibly by up to 50% by 2020, especially given the dramatic rise in type II diabetes. In recent years, age-related macular degeneration (AMD) the leading cause of vision loss in the elderly, has been successfully treated in many patients with intravitreal injections of LUCENTIS® (ranibizumab) or AVASTIN® (bevacizumab). Such drugs can require regular, e.g., monthly, injections to maintain the improvement, costing tens of thousand of dollars annually. In addition, some studies have brought into question the safety of long term treatment with these drugs, finding that accumulation of the drug in higher doses can result in destruction of PRs. Other forms of neural blindness, such as Retinitis Pigmentosa and Stargardt Disease, cannot currently be treated by any available means.
A number of research projects have been undertaken to develop a retinal implant capable of restoring vision to patients suffering retinal diseases. Retinal, cortical and optic nerve visual prostheses use microfabricated electronic components to stimulate neural circuitry that is still available despite whatever neural damage has caused blindness. This approach is attractive in that prostheses can directly stimulate surviving nerve cells and uses the functionality of the remaining, largely intact retinal neuronal circuitry. However, despite decades of research, visual prostheses have not advanced beyond early clinical trials and have not yet produced a level of vision that has been demonstrated to improve the ability of patients to perform visual tasks related to daily activities.
The current state of the art for retinal prosthesis utilizes a camera to capture the image and then relay the neural stimulation parameters to a microelectrode array (MEA) implanted in proximity to retinal neurons. The MEA consists of metal electrodes of diameters on the order of 30 μm, which are embedded into a flexible material. This type of image acquisition and stimulation is being used by two leading groups in retinal implants—Second Sight, Inc. (Sylmar, Calif.), which target epi-retinal implant locations, and the Boston retinal implant project, which targets a sub-retinal implant location. The epi-retinal approach places electrodes in the vitreous fluid, attached to the surface of the retina, while the subretinal approach places electrodes on the outside of retina, wedged between the photoreceptors and the retinal pigment epithelium. The retina section in FIG. 1 shows the electrode positions for the two types of retina prostheses.
The number of electrodes required to yield various levels of visual acuity has been estimated to be within the range of 256 to 625 electrodes, which theoretically might yield best visual acuity of 20/240 and 20/30, respectively. The high density of ganglion cells in the retina suggests that a greater number of stimulating electrodes could be implanted in a given area. However, the number of electrodes required depends on the ability of the materials to safely transmit charge and on the proximity of the target tissue to those electrodes. The current technology is not yet capable of restoring vision to a level that is sufficient for patients to lead an independent life and perform regular daily activities.
The barriers to restoring vision to the blind are significant. In addition to biomaterial issues such as toxicity, tissue encapsulation and cellular/immune responses that might be triggered by foreign materials, an electrical prosthesis must also provide long-term stability of the metal electrodes while minimizing any tissue damage that occurs as a result of the electrical stimulation. Induced tissue damage will reduce the excitability of the tissue and limit the potential for vision restoration. The potential biocompatibility and long-term functional stability of a retinal prosthesis are further complicated by ongoing anatomical and physiological changes that inevitably occur within the retina in patients with retinitis pigmentosa, the primary disease that has been targeted by early visual prosthetic implantations.
As is known in the art, when particles of materials are created with dimensions of around 1-10μ, the material's properties change. As used herein, a “nanomaterial” is a material in which quantum effects rule the behavior and properties of particles. When particle size is made to be nanoscale, properties such as melting point, fluorescence, electrical conductivity, magnetic permeability, and chemical reactivity change as a function of the size of the particle. As used herein, a “nanodevice” is a device formed from nanomaterials. Nanodevices and nanomaterials can interact with biological systems at fundamental, molecular levels with a high degree of specificity. By taking advantage of this unique molecular specificity, these nanotechnologies can stimulate, respond to and interact with target cells and tissues in controlled ways to induce desired physiological responses, while minimizing undesirable effects.
Nanowires have been shown to function as phototransistors with high sensitivity. Due to the small lateral dimensions (100's of nm to 10's of μm) and large surface-to-volume ratio of silicon (Si) nanowires, the large number of states at a Si surface can trap carriers at the surface equivalence to a gate bias, resulting in phototransistive behavior that leads to high sensitivity. This unique property of Si nanowires makes these devices attractive for photodetection from ultraviolet to the near infrared. Zhang, A., et al. (“Silicon Nanowire Detectors Showing Phototransistive Gain”, Applied Physics Letters, 2008, Vol. 93, 121110-1-3) have shown that etched planar and vertical Si nanowires function effectively with gains exceeding 35,000 under low intensity UV illumination, demonstrating their potential for low light detection. The vertical Si nanowires in particular are effective at overcoming low physical fill factor (FF) limitations due to their strong waveguiding effects, which cause a large fraction of the photon energy to be funneled into the nanowires.